They are devided into two categories, consumer and business:
Consumer:
Vista Starter Edition
Vista Home Basic
Vista Home Premium
Business:
Business Edition
Enterprise Edition (only available under Volume License)
at the top, we have Ultimate Edition, which combines the best of both
worlds, no trade offs, you get everything in this, kit and caboodle.
The Consumer category has small limitations such as not being able to to
connect to domains, limited encryption capabilities, does not support,
features such as bitlocker, only available in the Business category and
Ultimate Edition. Starter and Home Basic, does not support the AERO Glass
effect or Tablet PC capabilities.
Business and Enterprise do not support Media Center.
To learn more, go to:
http://www.microsoft.com/windowsvista/versions